The invention relates to a base/bracket assembly to be used in a customized orthodontic apparatus for treating a patient and intended mainly, but not exclusively, to be used in the case of a lingual technique, that is to say with the apparatus located on the non-visible posterior face of the teeth.
Conventionally, such apparatuses comprise:                at least one orthodontic arch wire, in other words a metal wire which exerts on the teeth a force that tends to move them from their unsatisfactory initial position, known as the “malposition”, into a satisfactory final position, known as the “corrected position”;        and a set of brackets which are each provided with at least one groove for holding an orthodontic arch wire.        
The brackets are fixed individually to the patient's teeth in a predetermined position that allows the orthodontic arch wire to transfer to the teeth the necessary forces for them to pass from the malposition into the corrected position during the treatment. This fixing is produced by way of a “base”, that is to say an intermediate element, one face of which conforms to the surface of the tooth to which the bracket is intended to be fixed, and the other face of which supports the bracket.
Most commonly, one or more orthodontic arch wires, together with a single set of brackets each comprising one or more grooves, are used.
The development of lingual orthodontic techniques, which have the esthetic advantage of leaving the apparatus virtually invisible from the outside, started around 1980. In these techniques, an important element in the success of the treatment is the good positioning of the bracket and of its groove on the tooth, since this positioning determines the orientation of the forces that are imposed on the corresponding tooth and thus the orientation of the tooth in the various spatial directions when it is located in the final corrected position. This positioning is much more difficult to carry out in the lingual technique than in the technique known as labial or vestibular (in which the apparatus is located or the anterior face of the teeth) on account of the great angulation of the posterior faces of the teeth. This angulation means that a small error in the positioning of the bracket can place the groove in an incorrect position which is incapable of correcting the position of the tooth as desired.
IT techniques have been able to bring significant improvements to the ease of designing customized orthodontic apparatuses that are specific to each patient.
In particular, document WO-A-03/068099 teaches designing in a customized manner an assembly formed firstly by the virtual image of a base for fixing to the tooth, designed digitally on the basis of a computer image of the patient's arch with the malpositioned teeth, and secondly by a virtual image of a bracket provided with a groove for the insertion of the orthodontic arch wire, this image being drawn from a virtual library of brackets having predetermined forms. Next, a bracket formed of a single body resulting from the combination of these two images is produced by rapid prototyping. Next, an orthodontic arch wire that is shaped with the aid of a special device and is intended to connect the brackets and to bring the patient's teeth into the corrected position is designed.
Once the orthodontic arch wire has been inserted into the groove in the bracket, this groove is closed off by the practitioner so as to keep the arch wire there. Most commonly, this closing off is realized by inserting a metal or elastomeric ligature into the groove. However, this method has the drawback of not having optimum reliability on account of the fact that it is possible for the ligature not to be fitted correctly by the practitioner and its properties can change over time, in particular in the case of an elastomeric ligature. If the ligature is displaced or damaged, the patient has to go to the orthodontist's practice in order to have it replaced. Likewise, the careful realization of each ligature takes a great deal of time, and this makes the fitting of the arch wire a lengthy and uncomfortable procedure for the patient. Finally, if, during the course of the treatment, it is necessary to change the arch wire, the orthodontist has to irreversibly destroy the ligatures in order to remove the arch wire, this procedure also being lengthy and uncomfortable for the patient. This step of destroying the ligature also causes the orthodontist to exert forces that are transmitted to the bracket and can weaken its connection to the base.
Considerable progress has been made by using what are known as “self-ligating” brackets. These brackets are designed to hold an elastic (metal) clip which, once the arch wire has been inserted, is placed in a position which closes off the groove and allows the clip to prevent the arch wire from coming out. This placement is generally carried out by sliding the clip in the longitudinal direction of the bracket between a stand-by position leaving the groove open for the insertion of the arch wire and a closed functional position closing off the groove, it being possible for this sliding to be complemented by a rotational movement as in document U.S. Pat. No. 6,776,613. The clip is moved manually by the orthodontist using one of his usual tools and takes only a fraction of a second. Similarly, the disengagement of the clip prior to changing the arch wire is carried out by simply pulling in the opposite direction to the previous sliding and is just as quick. Finally, the mechanical and dimensional characteristics of the metal clip are stable over time and if the bracket and the clip are produced with good dimensional precision, the functionality and reliability of the ligature are satisfactory.
However, the known self-ligating brackets also have their drawbacks. They are often more bulky than ordinary brackets on account of the fact that the longitudinal sliding movement of the clip has to be able to take place over a certain travel. Above all, this sliding movement can be hampered if tartar has built up on the bracket because it has not been removed by proper brushing of the apparatus in the mouth by the patient. When the arch wire is changed, the opening and replacement of the clip are thus made more difficult and one of the advantages of self-ligating brackets is at least partially lost.